1. Field of the Invention
The present invention relates to a fuel cell anode, a membrane electrode assembly comprising the same, and a fuel cell using them. Further, the present invention also relates to a process for production of said fuel cell anode or membrane electrode.
2. Background Art
Polymer electrolyte fuel cells, in particular, using aqueous solutions of methanol as fuels have been vigorously studied to use as electric power supplies for mobile devices because they can work even at a low temperature and can be downsized and lightened. However, conventional fuel cells still have insufficient performance to be used widely.
A fuel cell is a device that converts chemical energy into electric energy by a catalytic reaction on the electrode. In the fuel cell, protons are generated on the anode catalyst and then transfer to the cathode through a proton-conductive polymer membrane. Therefore, for improving the performance of fuel cell, it is indispensable to develop a material having excellent proton-conductivity and to produce an electrode using that material.
Proton-conductive polymers containing perfluorosulfonic acid are well-known proton-conductive materials. For example, a tetrafluoroethylene/perfluorovinyl ether copolymer containing sulfonic acid groups as ion-exchange groups (e.g., Nafion [trademark], available from DuPont) is widely adopted as the proton-conductive material of the electrode catalyst layer. However, in view of realizing excellent proton-conductivity, there is still room for improvement in the electrode.
As a material for the electrode comprising a proton-conductive inorganic oxide and a proton-conductive organic polymer binder, a sulfuric acid-supported metal oxide (e.g., disclosed in JP-A 2004-158261 (KOKAI)) is known. The sulfuric acid-supported metal oxide is a proton-conductive inorganic oxide having solid super-strong acidity, and is prepared by the steps of: loading sulfate ion on the surface of oxide containing at least one element selected from the group consisting of zirconium, titanium, iron, tin, silicon, aluminum, molybdenum, and tungsten; and then heating to fix the sulfate ion on the surface of the oxide. Because of the fixed sulfate ion, the sulfuric acid-supported metal oxide can have proton-conductivity. However, since the sulfate ion are often released by hydrolysis, the sulfuric acid-supported metal oxide is an unstable proton-conductive material when used in a fuel cell, in particular, using a liquid fuel because the fuel cell generates water together with electric power. It is, therefore, presumed that there is much room for improvement in the sulfuric acid-supported metal oxide in consideration of ensuring stable electric power for a long time.
In JP-A 2006-32287 (KOKAI), the present inventors have already proposed a fuel cell electrode comprising oxide carriers such as oxide of Ti, oxide particles such as oxide of W, and a binder for uniting them. However, in the electrode catalyst layer comprising the oxides of Ti and W (proton-conductive inorganic oxides), PtRu catalyst and the binder, the binder is presumed to inhibit the correlations among the oxide super-strong acids or those among the PtRu catalyst carriers. Further, it is also presumed that an enough amount of water to generate protons cannot be supplied to the proton-conductive inorganic oxides or an enough amount of fuel cannot be supplied to the redox catalyst since the binder attaches onto the surfaces of the oxide super-strong acids or onto the surface of the redox catalyst, respectively. Consequently, it is presumed that a sufficient amount of three-phase interfaces, which are necessary for the electrode reaction, cannot be formed.
As described above, in order to improve the proton-conductivity of the electrode catalyst layer or to promote the electrode reaction, various proton-conductive materials have been developed. However, there is still room for improvement in the performance thereof.
When the aforementioned known proton-conductive inorganic oxides are synthesized, nanoparticles thereof are liable to aggregate and grow to lower the specific surface area and accordingly to decrease effective proton-conductive sites. As a result, the proton-conductivity is so lowered that the resultant fuel cell as a whole cannot provide sufficient power.